Composite structures of the type for military air vehicles are generally constructed from a standard set of product forms such as prepreg tape and fabric, and molded structures reinforced with woven or braided fabrics. These materials and product forms are generally applied in structural configurations and arrangements that mimic traditional metallic structures. However, traditional metallic structural arrangements rely on the isotropic properties of the metal, while composite materials provide the capability for a high degree of tailoring that should provide an opportunity for very high structural.
There is general confidence among the composite materials community that a high-performance all-composite lightweight aircraft can be designed and built using currently available manufacturing technology, as evidenced by aircraft such as the F-117, B-2, and AVTEK 400. However, composite materials can be significantly improved if an optimization tool is used to assist in their design. In the recent past, engineered (composite) materials have been rapidly developed [1-3]. Maturing manufacturing techniques can easily produce a large number of new improved materials. In fact, the number of new materials with various properties is now reported to grow exponentially with time [1].
Today an engineer has a menu of 40,000 to 80,000 materials at his/her disposal [4]. This means that material selection, for example when designing a new air vehicle, can be quite a difficult and complex task. On the other hand, the material that suits best the typical needs of a future air vehicle structure may still not be available. This is because new materials are currently developed based on standard material requirements rather than on those for future air vehicles. Therefore, two critical needs exist: 1) to develop an engineering tool that can assist designers in selecting materials efficiently in future air vehicle programs; 2) to develop a methodology that allows structural designers to design the material that meets best the lightweight and performance requirements of future air vehicle systems. A materials engineer will then identify the most suitable manufacturing process for fabricating such a material. This will ensure that the designer of future air vehicles is truly using the best material for his/her design, and that the new material developed by the materials engineer will meet the needs of the vehicle development program.
Topology optimization has been considered a very challenging research subject in structural optimization [5]. A breakthrough technique for the topology optimization of structural systems was achieved at the University of Michigan in 1988 [6], and it is known worldwide as the homogenization design method. In this approach, the topology optimization problem is transformed into an equivalent problem of “optimum material distribution,” by considering both the “microstructure” and the “macrostructure” of the structure at hand in the design domain. The homogenization design method has been generalized to various areas, including structural design and material design [7]. It has also been applied to the design of structures for achieving static stiffness [6, 8-9], mechanical compliance [10-12], desired eigenfrequencies [13-16], and other dynamic response characteristics [17-20]. By selecting a modern manufacturing process, new materials may become truly available, with tremendous potential applications. These examples demonstrate that the topology optimization technique can be used to design new advanced materials—materials with properties never thought possible. 1 Material density is defined as the ratio of the area filled with material to the area of the whole design domain.
In general, a main structure may have several functions: 1) support the weight of other vehicle structures, 2) resist major external loads and excitations, 3) absorb low-frequency shock and vibration, 4) manage impact energy. Also, the main structure in different parts of an air vehicle may play different roles, and the secondary structure of the air vehicle may in general have completely different functions, for instance ones related to aerodynamics, local impact, and isolation from high-frequency vibration and noise. Therefore, the materials used in the various parts of the vehicle need to be designed according to their primary functions.
Theoretically, an infinite number of engineered materials can be obtained through a given design process if no objective is specified for the use of the structure in the air vehicle system. In other words, engineered materials need to be designed in such a way that they are optimum for their functions in the air vehicle system and for the operating conditions they will experience.